Preheat oven to 350 degrees. Lightly spray a 12" baking dish (rectangular or oval), with non stick baking spray. (You can also use a 9x9" pan or baking dish.) Set dish aside.
Rinse and peel the sweet potatoes. Using a Chef's or Santoku knife, slice the peeled potatoes into 1/8" slices. Place the sliced potatoes on their side, keeping the potato shape, in the baking dish.
Make the brown sugar pecan crumble: to a bowl, add the brown sugar, flour, pecans, cinnamon, nutmeg and salt. Stir together with a fork to combine. Pour the melted butter over the top of the crumble and mix. Mixture will be crumbly and gravel-like.
Top the sweet potatoes with the pecan crumble mixture, placing the crumble on the tops and sides of the potatoes. Cover the baking dish with foil. (Covering the dish with foil is important, as it will prevent the pecans from over-browning and burning.)
Bake at 350 degrees for 35 minutes. Remove the foil and bake for another 10 minutes to toast the pecans. (Total bake time will be 45 minutes.)
Remove from oven and let cool for 10 minutes before serving.